Investigators probing the massive bushfire in Yosemite National Park are looking into whether an illegal marijuana farm may have triggered the blaze.

As the vast bushfire continued to rage in the iconic Californian tourism destination, several reports quoted a local fire chief who suggested marijuana growers may be to blame.

Todd McNeal, a fire chief in Twain Harte, one of the towns affected by the 88,630-hectare inferno, said investigators had not pinpointed the cause of the blaze.

"We don't know the exact cause," Mr McNeal was quoted as telling a community meeting.

However, he added it was "highly suspect that there might have been some sort of illicit grove, a marijuana-grow-type thing".

"We know it's human caused. There was no lightning in the area," he said.

US Forest Service officials say the cause of the fire remains under investigation.

The San Jose Mercury News reported that authorities in California have faced increasing problems with marijuana farms hidden deep in the region's rugged wilderness.

A 2009 fire that burned 90,000 acres in the Los Padres National Forest near Santa Barbara was triggered by a campfire at a marijuana farm.

Meanwhile, officials said Saturday they were optimistic of making further gains on the blaze, known as the Rim Fire, but warned that hot, dry conditions continued to create a challenging environment.

"We're hopeful that we are going to turn the corner, but it's hot, it's dry, and there is a westerly wind," US Forest Service spokeswoman Leslie Auriemmo said.

"There's a lot of fuel out there. We remain in a high state of alert."

Fire crews gradually containing fire

According to latest figures early Saturday, the fire has burned 888 square kilometres and continues to threaten 4,500 structures.

A total of 4,995 firefighters have been deployed to battle the flames, which have so far destroyed 11 homes and 97 outbuildings.

The fire, which started on August 17, was 35 per cent contained as of Saturday, up from 32 per cent on Friday.

Yosemite National Park officials insisted on Friday that the fire posed no threat to tourists heading to the landmark destination on a busy US holiday weekend.

The flames remain some 24 kilometres from Yosemite Valley, the heart of the park where millions of visitors flock every year to see majestic scenery such as the Half Dome and El Capitan rock formations.

"The area where it's burning right now is mostly wilderness... There's nothing in that location that would potentially be a safety issue," said Yosemite spokeswoman Kari Cobb.